I have a requirement that in process tracking screen, some custom column (e.g instruction) should be added so that different processes listed there can easily be differentiated.
I tried to do this change in Workspace through Preferences->Manage Column Headings -> Tracking. But it affects only the Tasks view (Audit tab) for the process and not the initial process tracking screen. I also tried creating a process variable and made it Visible in UI and Searchable so that it is shown in the column heading list. But still that heading does not show up in the initial process tracking screen.

Is there way through which we may be able to customize the column headers for the initial tracking screen?

Not that I'm aware of.
However, we have built an enhanced tracking and enquiry tool that provides a lot more flexibility than the built-in tracking, and is also very easy to configure.
Read about it: http://www.avoka.com/avoka/enquirytool.shtml
Try it: http://www.avoka.com/apps/checkcookie?location=/apps/qpacdownload&qpac=y&qpac_code=Worksp aceEnquiryTool1.8
Howard
http://www.avoka.com